If you are creating your PDF in InDesign, and you're generally happy with the results - except for the file size - you should consider using Acrobat to optimize the PDF.
See this page for details. You can control resolution of images, remove unneeded items, and audit the PDF to see how much space each item is using:
In addition to what Michael said, you might ask on the Acrobat forum how to make a PDF smaller. It has to do with embedding fonts, dowsizing pictures, etc. If you use one font your file will be smaller than if you use 10 or 20, for example.
A pdf is a flexible file format but so full of settings and embedding options, it can be tricky to make a very lean file.
I believe I've had best results with the rather sluggish old school way of making a pdf by exporting a Postscript file from Indesign and generating a pdf rom that with Adobe Distiller.
But with normal pdf exporting I usually get ok results like this:
- start with [High quality Print] setting
- In general tab> options, deselect 'Create tagged pdf'
- In comperssion tab, turn jpg compression from max to high
- In comperssion tab, turn bicubic downsampling to 160dpi, for images above 160dpi
- Do same for color and greyscale images
- Save and see if it turns out small enough and turn dpi lower if not happy yet.
And yes, the optimize feat in Adobe Acrobat can be helpful too, but biggest step can already be made trough the export.